1. Field of the Invention:
This invention relates to fibrillated weatherproof webs which have a network structure and are resistant to light and the like. The webs are each composed of a composite film of at least two layers, one being a first layer (I) extruded from a first thermoplastic resin and the other a second layer (II) extruded from a second thermoplastic resin having a melting point lower than the first thermoplastic resin and containing an effective amount of a light resistance imparting agent. The invention is also concerned with nonwoven fabrics using the webs. The invention further pertains to processes for the production of the webs and nonwoven fabrics.
2. Description of the Related Art:
Nonwoven fabrics are currently produced by various processes, among which the spun-bonding process may be mentioned as a typical example. This spun-bonding process comprises introducing bundles of unstretched melt-spun filaments into an aspirator jet, drawing and jetting them by compressed air, stretching them and at the same time imparting static charges to them, opening them into monofilaments by static repulsion, and collecting the resultant monofilaments on a conveyor having opposite charges. Many techniques have heretofore been proposed with respect to this process [for example, Japanese Patent Publication No. 4993/1962, Japanese Patent Publication No. 26599/1968, Japanese Patent Publication No. 14913/1969, Japanese Patent Publication No. 21817/1969, Japanese Patent Publication No. 1941/1970, Offlegungschrift 17 60 730 (Japanese Patent Publication No. 1942/1970), U.K. Patent Specification 1,219,921 (Japanese Patent Publication No. 10779/1970), U.K. Patent Specification 1,214,509 (Japanese Patent Publication No. 33876/1970), and U.K. Patent Specification 1,271,844 (Japanese Patent Publication No. 3317/1971)].
Further, nonwoven fabrics composed of at least one layer made of a fibrous material and at least one warped stretched tape laminated together are also known (U.S. Pat. No. 3,734,812).
As a further production process, nonwoven fabrics are formed by forming tubular films of a molten resin, stretching and fibrillating the films into fibrillated films having a network structure, i.e., into split fibers, spreading the split fibers over a predetermined width and setting them, and then laminating them in a warp/weft relationship [U.S. Pat. No. 3,669,795 (Japanese Patent Publication No. 2786/1972, Japanese Patent Publication No. 4738/1972 and U.K. Patent Specification 1,386,438). These nonwoven fabrics are used as padding cloths for dress fabrics, filters, gauzes, linens, masks, mining and engineering materials such as battery separators, agricultural and horticultural materials, building and construction materials, etc.
It is well-known that these plastic-made non-woven fabrics are exposed to sunlight, rain, air oxygen, air ozone, mould and the like and are hence deteriorated when employed outdoors.
As a measure especially against ultraviolet rays, heat, oxygen including ozone, and the like, it is the general practice to incorporate a light resistance imparting agent such as an ultraviolet absorber or light stabilizer or a weatherproofing agent such as an antioxidant in resins or to coat resins with such a material.
The following exemplary methods may be mentioned for the practice of such a measure:
A weatherproofing agent is kneaded directly into a resin. As in Japanese Patent Publication No. 2344/1961, an organic polymer film is immersed in a solution of an ultraviolet absorber and after evaporating the solvent, the thus-immersed film is heated to have the ultraviolet absorber united to the surface layers of the film.
As in U.K. Patent Specification 1,124,433 (Japanese Patent Publication No. 24160/1971), an ultraviolet absorber is dissolved in a liquid polyhydric alcohol and the coated.
Japanese Patent Application Laid-Open No. 43972/1988 discloses the coating of a weatherproof paint.
Japanese Patent Application Laid-Open Nos. 201237/1987 and 234924/1987 disclose a laminate formed by providing transparent films on outer sides and as a middle layer an adhesive layer added with an ultraviolet absorber.
Japanese Patent Application Laid-Open No. 238741/1987 discloses a film formed by providing a coating of an acrylic resin on one side or both sides of a polyvinyl chloride resin film containing an ultraviolet absorber.
In Japanese Utility Model Publication No. 37729/1987, is discloses a film formed by providing an acrylic resin coating, in which an ultraviolet absorber is incorporated, on one side of a biaxially-stretched polyethylene terephthalate film.
A multilayer resin film composed of a biaxially-stretched film as a base layer and a surface layer composed of a uniaxially-stretched film, which contains a weatherproofing agent, and laminated on the base layer is proposed in Japanese Patent Publication No. 59668/1987.
However, the method in which a weatherproofing agent or the like is directed kneaded in a resin requires the uniform incorporation of the weatherproofing agent or the like in a large amount. This method is therefore accompanied by drawbacks such that the resulting product is costly and when the product is a film, its uniaxial stretching results in stretch irregularity and/or cutting.
On the other hand, the incorporation of a large amount of an additive or filler in a base material layer to be stretched impairs the quality of the base layer and in addition, the large amount of the additive undergoes blooming to form die deposit in a stretching step. These lead to the occurrence of cutting or slitting upon stretching. Further, the white powder and die deposit blocks splitter blades so that splitting is hampered.
When a film defining slits, such as a nonwoven fabric of split fibers, is stretched in particular, the above problems become remarkable so that the above method is not suitable for actual production of nonwoven fabrics of split fibers.
Further, the coating of a pre-stretched film with a solution of an ultraviolet absorber, the coating of at least one surface of a base material with an acrylic resin containing an ultraviolet absorber, the lamination of at least one stretched film with at least one stretched film containing an ultraviolet absorber, and the like all contain irksome steps.
It may be contemplated of applying these methods to nonwoven fabrics of split yarns. Stretching of the film, which has been coated in advance with the solution of the ultraviolet absorber or with the acrylic resin containing the ultraviolet absorber, after formation of slits therein however leads to a problem such that the resulting film may be peeled off.
On the other hand, in the method in which the solution of the ultraviolet absorber or the acrylic resin containing the ultraviolet absorber is coated subsequent to stretching of a slitted film, the solution of the ultraviolet absorber or the acrylic resin containing the ultraviolet absorber is wasted and moreover, the adhesion between the stretched base material and the coating film is poor. Moreover, the light imparting effect is suspicious.
Furthermore, the conventional methods are accompanied by various other problems such as environmental solvent contamination, safety problems and management of solvents.